The Prefilter Set are the filters that run prior to the membrane of a RO or RO/DI system. The set includes one 5 micron 10" Sediment and two 5 micron 10" Carbon Block filters. These should be replaced every six months like clockwork. 

Water flows first through the sediment filter, then into the two carbon blocks, and then continues into the membrane of the RO system. 

Melev's Tip: When replacing the carbon prefilters, be sure that the used flat O-rings are removed as well.  New carbon filters come with new O-rings on each end, and if an old one is still in the bottom of the acrylic housing, the filter becomes too tall preventing the housing from sealing properly against the threaded cap of the RO system.